1. Field of the Invention
The present invention relates to alloys for use in manufacturing or fabricating implantable medical devices, and more particularly, to implantable medical devices manufactured or fabricated from alloys that are magnetic resonance imaging compatible.
2. Discussion of the Related Art
Percutaneous transluminal angioplasty (PTA) is a therapeutic medical procedure used to increase blood flow through an artery. In this procedure, the angioplasty balloon is inflated within the stenosed vessel, or body passageway, in order to shear and disrupt the wall components of the vessel to obtain an enlarged lumen. With respect to arterial stenosed lesions, the relatively incompressible plaque remains unaltered, while the more elastic medial and adventitial layers of the body passageway stretch around the plaque. This process produces dissection, or a splitting and tearing, of the body passageway wall layers, wherein the intima, or internal surface of the artery or body passageway, suffers fissuring. This dissection forms a “flap” of underlying tissue, which may reduce the blood flow through the lumen, or completely block the lumen. Typically, the distending intraluminal pressure within the body passageway can hold the disrupted layer, or flap, in place. If the intimal flap created by the balloon dilation procedure is not maintained in place against the expanded intima, the intimal flap can fold down into the lumen and close off the lumen, or may even become detached and enter the body passageway. When the intimal flap closes off the body passageway, immediate surgery is necessary to correct the problem.
Recently, transluminal prostheses have been widely used in the medical arts for implantation in blood vessels, biliary ducts, ureters, or other similar organs of the living body. These prostheses are commonly referred to as stents and are used to maintain, open, or dilate tubular structures. An example of a commonly used stent is given in U.S. Pat. No. 4,733,665 to Palmaz. Such stents are often referred to as balloon expandable stents. Typically the stent is made from a solid tube of stainless steel. Thereafter, a series of cuts are made in the wall of the stent. The stent has a first smaller diameter, which permits the stent to be delivered through the human vasculature by being crimped onto a balloon catheter. The stent also has a second, expanded diameter, upon application of a radially, outwardly directed force, by the balloon catheter, from the interior of the tubular shaped member.
However, one concern with such stents is that they are often impractical for use in some vessels such as the carotid artery. The carotid artery is easily accessible from the exterior of the human body, and is close to the surface of the skin. A patient having a balloon expandable stent made from stainless steel or the like, placed in their carotid artery, might be susceptible to severe injury through day-to-day activity. A sufficient force placed on the patient's neck could cause the stent to collapse, resulting in injury to the patient. In order to prevent this, self-expanding stents have been proposed for use in such vessels. Self-expanding stents act like springs and will recover to their expanded or implanted configuration after being crushed.
The prior art makes reference to the use of alloys such as Nitinol (Ni—Ti alloy), which have shape memory and/or superelastic characteristics, in medical devices, which are designed to be inserted into a patient's body, for example, self-expanding stents. The shape memory characteristics allow the devices to be deformed to facilitate their insertion into a body lumen or cavity and then be heated within the body so that the device returns to its original shape. Superelastic characteristics, on the other hand, generally allow the metal to be deformed and restrained in the deformed condition to facilitate the insertion of the medical device containing the metal into a patient's body, with such deformation causing the phase transformation. Once within the body lumen, the restraint on the superelastic member can be removed, thereby reducing the stress therein so that the superelastic member can return to its original un-deformed shape by the transformation back to the original phase.
One concern with self-expanding stents and with other medical devices formed from superelastic materials, is that they may exhibit reduced radiopacity under X-ray fluoroscopy. To overcome this problem, it is common practice to attach markers, made from highly radiopaque materials, to the stent, or to use radiopaque materials in plating or coating processes. Those materials typically include gold, platinum, or tantalum. The prior art makes reference to these markers or processes in U.S. Pat. No. 5,632,771 to Boatman et al., U.S. Pat. No. 6,022,374 to Imran, U.S. Pat. No. 5,741,327 to Frantzen, U.S. Pat. No. 5,725,572 to Lam et al., and U.S. Pat. No. 5,800,526 to Anderson et al. However, due to the size of the markers and the relative position of the materials forming the markers in the galvanic series versus the position of the base metal of the stent in the galvanic series, there is a certain challenge to overcome; namely, that of galvanic corrosion. Also, the size of the markers increases the overall profile of the stent. In addition, typical markers are not integral to the stent and thus may interfere with the overall performance of the stent as well as become dislodged from the stent.
A concern with both balloon expandable and self-expandable stents is magnetic resonance imaging compatibility. Currently available metallic stents are known to cause artifacts in magnetic resonance generated images. In general, metals having a high magnetic permeability cause artifacts, while metals having a low magnetic permeability cause less or substantially no artifacts. In other words, if the stent or other medical device is fabricated from a metal or metals having a low magnetic permeability, then less artifacts are created during magnetic resonance imaging, which in turn allows more tissue in proximity to the stent or other medical device to be imaged.
Artifacts created under magnetic resonance imaging are promoted by local magnetic field inhomogeneities and eddy currents induced by the magnetic field generated by the magnetic resonance imaging machine. The strength of the magnetic field disruption is proportional to the magnetic permeability of the metallic stent or other medical device. In addition, signal attenuation within the stent is caused by radio frequency shielding of the metallic stent or other medical device material. Essentially, the radio frequency signals generated by the magnetic resonance imaging machine may become trapped within the cage like structure of the stent or other medical device. Induced eddy currents in the stent may also lead to a lower nominal radio frequency excitation angle inside the stent. This has been shown to attenuate the signal acquired by the receiver coil of the magnetic resonance imaging device. Artifact related signal changes may include signal voids or local signal enhancements, which in turn degrades the diagnostic value of the tool.
Accordingly, there is a need to develop materials for implantable medical devices, such as stents, that are magnetic resonance imaging compatible while retaining the toughness, durability and ductility properties required of implantable medical devices such as stents.